Chapter 1: Where to start?
If a JW says to you: "We have the truth and you know it!". How could you reply?
You could say:"I know many JW's and they are very sincere in their beliefs. But don't you think everyone deserves to hear the truth even if it contradicts something they truly believe?" If the JW agrees you could share with them scriptures that highlights Jesus warnings about false prophets and false shepherds who would seek to plunder and destroy the flock for their own glory and gain. (Luke 21:8, John 10:1,2, Matthew 7:15).
You might then arrange to come back and discuss the topic further.